The majority of present day integrated circuits are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs), or simply MOS transistors. A MOS transistor includes a gate electrode as a control electrode and spaced apart source and drain regions between which a current can flow. A control voltage applied to the gate electrode controls the flow of current through an underlying channel between the source and drain regions.
Magnetic random access memory (MRAM) is a nonvolatile memory technology that uses magnetization to represent stored data. Generally, MRAM includes a plurality of magnetic cells in an array. Each cell typically represents one bit of data, and thus may be referred to as a “bit cell.” Included in the cells are magnetic elements. A magnetic element may include two ferromagnetic “plates” (or layers upon a semiconductor substrate) each of which has a magnetization direction (or orientation of magnetic moments) associated with it. The two ferromagnetic plates are separated by a thin non-magnetic layer. One specific type of MRAM element is a magnetic tunnel junction (MTJ) element. An MTJ element includes at least three basic layers: a “free layer,” a tunneling barrier layer, and a “fixed layer.” The free layer and the fixed layer are ferromagnetic layers; the tunneling barrier layer is a thin insulator layer located between the free layer and the fixed layer. The magnetization direction of the free layer is free to rotate, either in the film plane or perpendicular to the film plane; the magnetization of the fixed layer is fixed in a particular direction typically co-linear to the free layer. A bit is written to the MTJ element by orienting the magnetization direction of the free layer in one of the two directions. Depending upon the orientations of the magnetic moments of the free layer and the fixed layer, the resistance of the MTJ element will change. Thus, the bit may be read by determining the resistance of the MTJ element. When the magnetization direction of the free layer and the fixed layer are parallel and the magnetic moments have the same polarity, the resistance of the MTJ element is low. Typically, this is designated a “0.” When the magnetization direction of the free layer and the fixed layer are anti-parallel and the magnetic moments have the opposite polarity, the resistance of the MTJ is high. Typically, this is designated a “1.”
Spin torque transfer (STT) (also known as spin transfer switching or spin-transfer effect) is one technique for writing to MTJ memory elements. STT is based upon the idea that when a spin-polarized current (most of the electrons of the current have spins aligned in the same direction) is applied to a free ferromagnetic layer, the electrons may transfer their spin angular momentum to the free layer to switch the direction of magnetization of the free layer. The advantages of using STT for writing to magnetic elements include smaller bit size and lower writing current requirement as compared with its predecessor toggle MRAM which using magnetic field for write operation. One of the advantages of STT-MRAM, or more specifically the recent perpendicular MRAM is its scalability, i.e. MRAM can be made smaller such that memory capacity is higher. However, scaling of MRAM memory requires miniaturization of the transistor together with MRAM. Currently one of the challenges in MRAM is that the select transistor may not be able to supply enough current due to its small size to switch the MTJ element reliably. Typically higher the current, more reliable is the switching of MTJ between 0 and 1 state. Moreover, in STTMRAM the switching current (or “drive” current) required to switch the magnetization direction of the MTJ element from parallel to anti-parallel is about 20 to about 50 percent larger than that required to switch from anti-parallel to parallel. Furthermore, in a conventional STT MTJ element the larger parallel-to-anti-parallel switching current is further limited by a “source degeneration” or the so called “source-site loading” effect. This source degeneration effect constrains the amount of drive current flowing through the MTJ element and may prevent the MTJ element from switching the magnetization direction from anti-parallel to parallel reliably. The reader may reference U.S. Pat. No. 8,416,600 B2 (Lin et al.) for further information regarding STT MRAM structures, as well as prior art attempts to achieve more reliable parallel-to-anti-parallel switching.
Accordingly, it is desirable to provide an MRAM structure with drive current amplification to ensure reliable switching of the magnetization direction of the MTJ element from parallel to anti-parallel. Additionally, it is desirable to provide methods for the fabrication of such structures that are easily integrated into existing process flow schemes used in semiconductor fabrication facilities. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.